OpenActive Beta Namespace

Hi all,

Following on from discussions with many of you during implementation, allow
me to propose a beta namespace.

This enables us all to add new properties to our endpoints so that we can
publish all data fields we have that are useful, while providing a forum
for discussion around each new property. The idea being that these
properties would be considered by the group for inclusion in the
specification.

We can easily add the machine-readable JSON-LD behind this, but in the
first instance see the this page <https://www.openactive.io/ns-beta/>.

Note the issue associated with each one to facilitate discussion. To
suggest a new property simply create a new issue here
<https://github.com/openactive/ns-beta/issues>, and optionally (if you're
feeling techy) create a pull request to add it to the schema.

I know we are converging on Version 1 of the schema, and it would be great
to get these out of beta for that if possible! Especially those properties
used by leisure operators.

Many thanks,

Nick